With the increasing down-scaling of integrated circuits and the increasingly demanding requirements to the speed of integrated circuits, transistors need to have higher drive currents with increasingly smaller dimensions. Fin Field-Effect Transistors (FinFETs) were thus developed. The FinFETs include vertical semiconductor fins. The semiconductor fins are used to form source and drain regions, and to form channel regions between the source and drain regions. Shallow Trench Isolation (STI) regions are formed to define the semiconductor fins. The FinFETs also include gate stacks, which are formed on the sidewalls and the top surfaces of the semiconductor fins.
In the formation of the STI regions, a pad oxide layer and a nitride hard mask may be used to define the patterns of the STI regions. The pad oxide layer and the nitride hard mask are first etched using a patterned mandrel structure as an etching mask, and then the semiconductor substrate is etched using the nitride hard mask as another etching mask to form trenches in the semiconductor substrate. Trenches are then filled with a dielectric material to form STI regions.